1. Field of the Invention
The invention relates in general to a touch panel, and more particularly to a method for detecting touch-point coordinate for use in a resistive touch panel.
2. Description of the Related Art
The Personal Digital Assistance (PDA) becomes more and more popular than the conventional notebook due to its portability and multi-function.
A typical PDA 100 is shown in FIG. 1. The PDA 100 includes a base 102, a touch panel 104, a control switch 106, and a stylus 108, wherein the touch panel 104 is positioned on the top surface of the base 102. The user can operate the PDA 100 by touching the touch panel 104 with stylus 108.
The touch panels can be categorized into two types: capacitive type and resistive type. FIG. 2 illustrates a resistive touch panel formed with an X plate 202 and a Y plate 204 being disposed apart. Both the X plate 202 and the Y plate 204 are plane resistors and are not in contact with each other if no external force is exerted on the touch panel. As the user points the stylus to the touch panel, the point P1 of the X plate 202 is attached to point P2 of the Y plate 204, wherein the touched point on the touch panel corresponds to the point P1 on the X plate 202 and the point P2 on the Y plate.
Please refer to FIG. 3; it shows the equivalent circuit of the resistive touch panel, wherein the equivalent resistor RM is produced between the X plate and the Y plate. When no external force is exerted on the touch panel, an open circuit is formed between the X plate and the Y plate, and the resistance of the equivalent resistor RM is infinity. When the X plate is attached to the Y plate by an external force, a loop is formed across the X plate and the Y plate, and the equivalent resistor RM is much smaller than infinity, wherein the equivalent resistor RM relates to the magnitude of the external force. Moreover, equivalent resistors Rxo and Rx1 on the X plate are coupled to the point P2, wherein the magnitude of the Rxo and Rx1 are determined by the X-coordinate of the point P2, not the Y-coordinate of the point P2. Likewise, the two equivalent resistor Ryo and Ry1 on the Y plate are coupled to the point P1, wherein the magnitude of the Ryo and Ry1 are determined by the Y-coordinate of the point P1, not the X-coordinate of the point P2.
Generally, a touch-point detecting apparatus is employed to determine the coordinates of the touched point on the touch panel. If the stylus touches the touch panel too lightly, the contact status between the Y plate and the X plate will be unstable, and the corresponding equivalent resistor RM will be different with that resulting from normally pressed. In this case, the detection result is in error and accordingly may cause the PDA to operate in wrong status.